1. Field of the Invention
This invention relates generally to modulator drivers and, more particularly, to a modulator bias control including a broadband Bias-T network.
2. Description of the Related Art
Modulator drive circuits have become commonplace in telecommunication systems. Generally, such a modulator drive circuit accepts a data signal as an input, e.g. a data signal to be transmitted over a network infrastructure, and generates a corresponding modulator drive signal to be provided to a modulator, e.g. a Mach-Zehnder modulator (MZM) or a semiconductor electro-absorption modulator (EAM). In response to the output modulator drive signal the modulator then modulates an optical carrier to facilitate optical transmission of the data signal across the network infrastructure. The modulator drive signal provided to the modulator generally comprises two signal components, a first signal component which represents the data waveform to be transmitted across a network infrastructure and a second signal component which is used to bias the modulator to ensure that the modulator is operating to efficiently modulate the optical carrier. The data waveform is characterized by a defined amplitude and a random sequential bit sequence corresponding to the data, while the second signal component is often referred to as the DC bias.
With reference to FIG. 1, a first known modulator drive circuit 100, which utilizes a bias circuit in the signal path, is shown. More specifically, drive circuit 100 comprises a high speed buffer 110, and a bias circuit 112, also referred herein as Bias-T circuit 112, located external to the high speed buffer 110 and in the signal path of a modulator driver output signal 108. The high speed buffer 110 is configured to accept an input data signal 104, labeled “Vin” in FIG. 1. Although not necessary, typically the input data signal 104 is a differential signal as depicted. The high speed buffer 110 generates an output signal 106 in response to the received input data signal 104, the output signal 106 provided to the bias circuit 112. The primary purpose of the high speed buffer 110 of FIG. 1 is to transform the differential input signal 102 into a signal having a sufficient amplitude to drive a modulator 180, for example a Mach-Zehnder modulator or an electro-absorption modulator. Additionally, the primary purpose of the Bias-T circuit 112 is to provide the modulator 180 with a suitable bias level for efficient optical modulation. For clarity, the output signal 106 is depicted as a single-ended signal, however output signal 106 could be a differential signal including a first signal portion and a second signal portion, each portion applicable to the following discussion herein. The high speed buffer 110 is powered via a fixed voltage differential power supply which provides an upper fixed voltage of V1 and a lower fixed voltage of V2 to high speed buffer 110, as shown. The fixed voltage supply provides a suitable voltage range, V1-V2, to accommodate the amplitude of the differential input data signal 104.
The bias circuit 112 comprises a capacitor 112C, an inductor 112L, and an adjustable DC bias voltage input, labeled DC BIAS in FIG. 1. The output signal 106 of the high speed buffer 110 is AC coupled through the capacitor 112C and passed on to the modulator 110 as a data waveform of a modulator drive signal 108. Additionally, capacitor 112C correspondingly blocks any DC voltage signal which may exist as part of output signal 106, preventing the DC signal from entering the Bias-T 112. The bias signal voltage, labeled “DC BIAS”, is coupled through the inductor 112L and passed on to the modulator 180, the capacitor 112C also preventing the bias signal voltage from exiting the Bias-T 112 and entering the high speed buffer 110. Additionally, the inductor 112L prevents the bias circuit which develops the DC BIAS (not shown) from loading the high speed buffer 110, thus the output signal 106 passes through the capacitor 112C and straight on to the modulator 180.
Such a circuit 100, however, has several drawbacks. First, the Bias-T circuit 112 has a desired frequency range of operation limited by the selected capacitor 112C and inductor 112L utilized. Typically the capacitor 112C and inductor 112L are selected to provide a low-pass cut-off frequency which is lower than the low-pass cut-off frequency of the spectrum of the data waveform at the output of the high speed buffer 110. Thus, the response of the cascaded high speed buffer 110 and the bias-T circuit 112 will not be broadband if the data waveform spectrum extends below the Bias-T 112 cut-off frequency, leading to undesirable distortion in the modulator output signal during broadband operation.
Second, with the frequency range of a broadband modulator driver being inversely proportional to the impedance of the capacitor 112C, a broadband modulator driver requires relatively large physical die areas for the components of the Bias-T 112. This problem is exacerbated by the fact that the modulator drive circuit 100 may be one of a plurality of modulator drive circuits, where it is desired to provide the plurality of modulator drive circuits on a single substrate as part of a semiconductor integrated circuit chip. Moreover, it is desirable to reduce the distance of the signal path between the high speed buffer 110 and the modulator 180 to correspondingly reduce, or otherwise minimize, the associated transmission line effects.
What is needed is a broadband modulator drive circuit which generates a proper modulator drive signal over a broad range of operating frequencies. Also, what is needed is a broadband modulator drive circuit which includes a broadband Bias-T circuit which provides a proper modulator drive signal regardless of the frequency range of the data waveform. What is further needed is a broadband Bias-T circuit providing for an adjustable bias level. Still, what is needed is a modulator drive circuit which includes circuitry to allow for correcting response errors due to fabrication tolerances. Last, what is needed is the ability to provide an array of such modulator driver circuits on a single substrate.